This application relates to select switches used in three-dimensional structures such as memory arrays, to methods of making and using such switches, and structures containing such switches.
Uses of re-programmable non-volatile mass data storage systems utilizing flash memory are widespread for storing data of computer files, camera pictures, and data generated by and/or used by other types of hosts. A popular form of flash memory is a card that is removably connected to the host through a connector. There are many different flash memory cards that are commercially available, examples being those sold under trademarks CompactFlash (CF), the MultiMediaCard (MMC), Secure Digital (SD), miniSD, microSD, Memory Stick, Memory Stick Micro, xD-Picture Card, SmartMedia and TransFlash. These cards have unique mechanical plugs and/or electrical interfaces according to their specifications, and plug into mating receptacles provided as part of or connected with the host.
Another form of flash memory systems in widespread use is the flash drive, which is a hand held memory system in a small elongated package that has a Universal Serial Bus (USB) plug for connecting with a host by plugging it into the host's USB receptacle. In yet another form of flash memory systems, a large amount of memory is permanently installed within host systems, such as within a notebook computer, tablet computer or similar device, as a Solid State Drive (SSD), in place of the usual disk drive mass data storage system. Each of these three forms of mass data storage systems generally includes the same type of flash memory arrays. They each also usually contain its own memory controller and drivers but there are also some memory only systems that are instead controlled at least in part by software executed by the host to which the memory is connected. The flash memory is typically formed on one or more integrated circuit chips and the controller on another circuit chip. But in some memory systems that include the controller, especially those embedded within a host, the memory, controller and drivers are often formed on a single integrated circuit chip.
Flash memory systems typically utilize integrated circuits with arrays of memory cells that individually store an electrical charge that controls the threshold level of the memory cells according to the data being stored in them. Electrically conductive floating gates are most commonly provided as part of the memory cells to store the charge but dielectric charge trapping material is alternatively used. A NAND architecture is generally preferred for the memory cell arrays used for large capacity mass storage systems. Other architectures, such as NOR, are typically used instead for small capacity memories. Examples of NAND flash arrays and their operation as part of flash memory systems may be had by reference to U.S. Pat. Nos. 5,570,315, 5,774,397, 6,046,935, 6,373,746, 6,456,528, 6,522,580, 6,643,188, 6,771,536, 6,781,877 and 7,342,279.
The amount of integrated circuit area necessary for each bit of data stored in the memory cell array has been reduced significantly over the years, and the goal remains to reduce this further. The cost and size of the flash memory systems are therefore being reduced as a result. The use of the NAND array architecture contributes to this but other approaches have also been employed to reducing the size of memory cell arrays. One of these other approaches is to form, on a semiconductor substrate, multiple two-dimensional memory cell arrays, one on top of another in different planes, instead of the more typical single array. Examples of integrated circuits having multiple stacked NAND flash memory cell array planes are given in U.S. Pat. Nos. 7,023,739 and 7,177,191.
Another type of re-programmable non-volatile memory cell uses variable resistance memory elements that may be set to either conductive or non-conductive states (or, alternately, low or high resistance states, respectively), and some additionally to partially conductive states and remain in that state until subsequently re-set to the initial condition. The variable resistance elements are individually connected between two orthogonally extending conductors (typically bit and word lines) where they cross each other in a two-dimensional array. The state of such an element is typically changed by proper voltages being placed on the intersecting conductors. Since these voltages are necessarily also applied to a large number of other unselected resistive elements because they are connected along the same conductors as the states of selected elements being programmed or read, diodes are commonly connected in series with the variable resistive elements in order to reduce leakage currents that can flow through them. The desire to perform data reading and programming operations with a large number of memory cells in parallel results in reading or programming voltages being applied to a very large number of other memory cells. An example of an array of variable resistive memory elements and associated diodes is given in U.S. Pat. No. 7,902,537.
Portions of a 3-dimensional memory array may be accessed using select switches that are controlled by select lines. In general, the size of such select switches is limited by the pitch of the memory array, which tends to get smaller from one generation to the next. On the other hand, select transistors may be required to provide larger currents as they connect ever-larger numbers of memory cells (e.g. as more cells are added in vertical stacks by increasing the number of layers in such vertical stacks). It is desirable to make select switches that can satisfy such conflicting demands by supplying a large current while occupying a small area on a die.